1. Field
The present invention relates to a wireless communication system, and more particularly to a method and an apparatus for generating a sequence used in a wireless communication system.
2. Discussion of the Background
In a communication system, a synchronization signal or a Synchronization CHannel (SCH) is essentially required for the detection of a sub-frame timing and a frame timing, the detection of a cell IDentification (ID), etc.
A Long Term Evolution (LTE) system, which is developed from a Wideband Code Division Multiple Access (WCDMA) system corresponding to the 3rd generation mobile communication standard, performs a process of searching for a cell as follows.
First, one of three different sequences, which is a primary synchronization signal (PSS), is transmitted on a cycle of 5 ms. These three sequences are defined as cell IDentifications (IDs) in a cell group, respectively. A frame timing and a cell group ID are detected from a secondary synchronization signal (SSS), and the number of cell groups is defined as 168. A cell group ID can also be detected through an orthogonal Reference Signal (RS) sequence. However, in a 3rd Generation Partnership Project (3GPP) LTE system, a cell group ID performs only the role of identifying a cell group ID detected in the previous process by using an orthogonal reference signal sequence. Thereafter, the LTE system decodes a Broadcast CHannel (BCH).
From the viewpoint of a search for an initial cell, the 3GPP LTE system must be capable of discriminating between a total of 504 base stations, and discriminates between the 504 base stations by using 168 cell groups and 3 cell IDs in each cell group. In this case, the 168 cell groups are detected through a secondary synchronization signal, and this detection is requires 168 or more different sequences, which can be mapped for the transmission of the secondary synchronization signal.
Meanwhile, the 3GPP LTE system uses a total of 6 resource blocks in a frequency domain to transmit a synchronization signal. Because each resource block includes 12 sub-carriers in a frequency domain, the maximum number of sub-carriers used for a synchronization signal is equal to 72 in total.
Therefore, a secondary synchronization signal sequence or a secondary synchronization signal code, which is mapped for the transmission of the secondary synchronization signal, must have a length or a period equal to or smaller than 72. The number (or size) of secondary synchronization signal sequences must be equal to or larger than 168. In order to show reliable performance, the secondary synchronization signal sequence must have a cross-correlation value between sequences which is equal to or smaller than a predetermined value.
Generally, the number of sequences depends on the length of the sequence. Namely, when a typical sequence has a length equal to or smaller than 72, The number of typical sequences is also equal to or smaller than 72. Accordingly, the typical sequences do not satisfy 504 or more, which is the number of different base stations between which a discrimination is made, required for a synchronization signal used to discriminate an initial cell from another cell. In order to solve this problem, a total of 504 or more base stations are divided into groups, and a is discrimination can be made between the groups on two steps by using a primary synchronization signal and a secondary synchronization signal. However, the primary synchronization signal has a disadvantage in that the overhead of a system exponentially increases when an excessive number of different sequences are used to detect a symbol timing. Only three sequences are used in consideration of this disadvantage. When only three sequences are used, 168 or more different sequences are required for the secondary synchronization signal.
At this time, when either a GCL (General Chip Like) sequence, a Zadoff-Chu sequence, a Hadamard sequence, or a binary sequence of m-sequence series, which is usually used for a conventional synchronization signal, has a length equal to or shorter than 72, the number of sequences is also equal to or smaller than 72. Accordingly, the sequences do not satisfy 168 or more, which is the number of sequences required for the secondary synchronization signal. In order to solve this problem, many methods have been proposed. Representative methods include a method for multiplying a sequence required for the secondary synchronization signal by a value, obtained by performing a cyclic shift or a rotation on a GCL to sequence or a Zadoff-Chu sequence, and increasing the number of sequences required for the secondary synchronization signal; and a method for interleaving two or more binary sequences, each of which has a shorter length than 72, and causing the number of sequences required for the secondary synchronization signal to be larger.
Evaluation results of simulations showed that among the methods as described is above, the method by which two binary sequences, each of which had a shorter length than 72, were interleaved and then a sequence obtained by the interleaving was used, showed better performance than those of the other methods. Finally, it was determined that this method was to be used for a sequence mapped to the secondary synchronization signal of the 3GPP LTE system.
Specifically, the prior art will be described below with reference to the accompanying drawings. FIG. 1 is a view of the configuration of a synchronization signal in a conventional 3GPP LTE system, taken from the viewpoint of time.
As shown in FIG. 1, one frame corresponding to 10 ms includes 10 sub-frames, and one sub-frame includes 2 slots. Each slot includes 6 symbols (when having an extended cyclic prefix) or 7 symbols (when having a normal cyclic prefix). At this time, on a cycle of 5 ms, a Primary Synchronization Signal (PSS) or a Primary Synchronization CHannel (P-SCH) is mapped to the last symbol of the first slot in the first sub-frame and the last symbol of the first slot in the sixth sub-frame of each frame, and is then transmitted through the last symbol of the first slot in the first sub-frame and the last symbol of the first slot in the sixth sub-frame of each frame.
Meanwhile, a Secondary Synchronization Signal (SSS) or a Secondary Synchronization CHannel (S-SCH) is mapped and is then transmitted, on a cycle of 10 ms. Specifically, a first signal (S-SCH1) of the Secondary Synchronization Signal (SSS) or the is Secondary Synchronization CHannel (S-SCH) is mapped to the second symbol after the first slot in the first sub-frame of each frame, and is then transmitted through the second symbol after the first slot in the first sub-frame of each frame. Also, a second signal (S-SCH2) of the secondary synchronization signal is mapped to the second symbol after the first slot in the sixth sub-frame of each frame, and is then transmitted through the second symbol after the first slot in the sixth sub-frame of each frame.
FIG. 2 is a view of the configuration of a synchronization signal in a conventional 3GPP LTE system, taken from the viewpoint of frequency. The 3GPP LTE system allows a sequence (or a code) for a synchronization signal to be mapped to a total of 72 sub-carriers located in the center among all sub-carriers of a relevant symbol for each synchronization signal. Actually, only 62 center sub-carriers of the 72 sub-carriers are used, and 5 sub-carriers at either end are reserved, and are not used. At this time, a sequence for the secondary synchronization signal is mapped to the 62 sub-carrier by using two different m-sequences all having a length of 31.
A specific mapping method is as follows. In the configuration of a first signal of the secondary synchronization signal as shown in FIG. 2, a first m-sequence is mapped to even-numbered sub-carriers, and a second m-sequence is mapped to odd-numbered sub-carriers. Also, in the configuration of a second signal of the secondary synchronization signal, in contrast, a first m-sequence is mapped to odd-numbered sub-carriers, and a second m-sequence is mapped is to even-numbered sub-carriers.
At this time, in the case of an m-sequence having a length of 31, a total of 31 different sequences exist. Accordingly, the number of combinations, which allow any two different sequences to be selected from among the 31 different sequences and enable mapping using the two selected different sequences, is equal to a total of 31×31=961. A result of a simulation showed that 168 combinations had the best performance among these 961 combinations. Accordingly it was finally determined that the 168 combinations were to be used in the 3GPP LTE system.
As described above, in order to discriminate between multiple pieces of cell specialization information including a cell ID, etc., which are sufficient, the 3GPP LTE system requires at least 168 or more as the number of different sequences for the secondary synchronization signal.
However, the number of sub-carriers to which a sequence can be mapped for the secondary synchronization signal in one symbol, is equal to a maximum of 72. Therefore, the length of the sequence is equal to or shorter than 72. It goes without saying that when two or more symbols are used, the length of a sequence which can be mapped increases by twice the number of symbols. However, when multiple symbols are used, there is a disadvantage in that overhead exponentially increases.
Therefore, the secondary synchronization signal sequence, which is mapped for is the transmission of the secondary synchronization signal, must have a length or a period equal to or shorter than 72. The number of secondary synchronization signal sequences must be equal to or larger than 168. In order to show reliable performance, the secondary synchronization signal sequence must have a cross-correlation value between sequences which is equal to or smaller than a predetermined value.
In this respect, when either a GCL (General Chip Like) sequence, a Zadoff-Chu sequence, a Hadamard sequence, or a binary sequence of m-sequence series, which is usually used for a conventional synchronization signal, has a length equal to or shorter than 72, the number of sequences is also equal to or smaller than 72. Accordingly, the sequences do not satisfy 168 or more, which is the number of sequences required for the secondary synchronization signal.
In order to solve this problem, a proposal was made of a method for multiplying a sequence required for the secondary synchronization signal by a value, obtained by performing a cyclic shift or a rotation on a GCL sequence or a Zadoff-Chu sequence, and increasing the number of sequences required for the secondary synchronization signal; or a method for interleaving two or more binary sequences, each of which has a shorter length than 72, and causing the number of sequences required for the secondary synchronization signal to be larger. Further, the number of combinations, which allow any two different sequences to be selected from among a total of 31 different m-sequences each having a length of 31 and enable mapping using the two selected different sequences, is equal to a total of 31×31=961. A result of a simulation showed that 168 combinations had the best performance among these 961 combinations. Accordingly it was finally determined that the 168 combinations were to be used in the 3GPP LTE system.
M-sequences have a very small cross-correlation value between m-sequences which are not interleaved, and thus have very good performances. However, due to a problem of a collision and ambiguity between sequences, a cross-correlation value between sequences each having a length of N generated by interleaving two different m-sequences each having a length of N/2 becomes larger than a cross-correlation value between m-sequences which are not interleaved, so as to cause performance degradation. Although the 168 combinations which have the least collision and ambiguity among the 961 combinations as described above are selected, a problem of performance degradation still exists as compared with a case where interleaving is not performed on sequences. A problem caused by a combination of these two short sequences for the secondary synchronization signal is usually called a nested problem.